1. Field of the Invention
The present invention relates to optical ramp filters. More specifically, the present invention relates to a system and method of fabricating high resolution optical ramp filters.
2. Description of the Related Art
Certain optical analytical systems require filter components, such as optical ramp filters, that provide precise values of optical transmission and reflectance that vary as a function of position on the filter. A detailed description of such a system is given in copending commonly assigned U.S. patent application Ser. No. 08/045,841, now U.S. Pat. No. 5,350,917 filed on Apr. 9, 1993, entitled "Wavefront Error Estimation Derived From Observation Of Arbitrary Unknown Extended Scenes".
In typical optical ramp filters, a thin layer reflective material, such as silver or gold, is deposited on a window that is optically transparent at the filter's operating wavelength. The thickness of the reflective layer is varied as a function of position on the window.
When light (at the filter design wavelength) is directed at the filter, a portion of this light is reflected by the reflective layer and a portion is transmitted through the reflective layer and the window. As the reflective layer thickness increases, there is a corresponding increase in the percentage of light that is reflected from the reflective layer. If the reflective layer exhibits no absorption and if it is formed on an ideal window that exhibits no absorption, the percentage of light transmitted (T) and the percentage of light reflected (R) are related by the expression: 1=T+R. Thus, as the percentage of light reflected increases, there is a corresponding decrease in the percentage of light transmitted.
Prior methods and systems for fabricating optical ramp filters vary the thickness of the reflective layer at the time that it is being deposited on the window. The reflective layer is typically deposited using well known evaporation or sputtering techniques, such as those explained in Kasturi L. Chopra, Thin Film Phenomena, McGraw-Hill, New York (1969), pages 10, 11, 21 and 35. One of the problems with these standard deposition techniques is the existence of "clustering" during the early stages of the deposition process.
As the metal is deposited onto the window, the metal deposits form separate isolated islands or clusters. When the layer begins to approach 4 to 8 nm in thickness, the separate metal clusters begin to merge. When the layer reaches 4 to 8 nm in thickness, the clusters completely merge to form a smooth metal layer.
As a result of this clustering effect, the resolution or control that can be achieved with respect to film thickness variation is limited. This results in less control and lower resolution with respect to the transmission and reflectance variation of the optical ramp filter. Since the clustering effect remains until the film is at least 4-8 nm thick, it is not possible to make very thin films (for example, films with a thickness that varies from 0 to 10 nm) or to vary the film thickness by less than 8 nm reliably. With evaporation or sputtering techniques, film thickness variations are typically 10 to 100 nm.